Anisotropic conductive sheets (also called anisotropic conductive films) having various structures are shown in the following references 1-9:
reference 1: JP-A-51-93393
reference 2: JP-A-53-147772
reference 3: JP-A-54-146873
reference 4: JP-B-56-48951
reference 5: JP-A-4-151889
reference 6: JP-A-7-105741
reference 7: JP-A-2000-133063
reference 8: JP-A-2001-185261
reference 9: U.S. Pat. No. 4,292,261
As shown in FIG. 6(a), in an anisotropic conductive sheet 100 comprising conductive particles 102 uniformly dispersed in an insulating resin sheet 101, respective conductive particles 102 are separated from each other in an unused (non-compressed) state. As shown in FIG. 6(b), however, when the anisotropic conductive sheet 100 is interposed between an electrode 111 of an electronic part 110 to be connected and a wire circuit 121 of a substrate 120 for mounting, and compressed from both sides, the conductive particles are contacted with each other in the compressed part to electrically conduct the electrode 111 and the circuit 121.
In the anisotropic conductive sheet shown in FIG. 6(c), (d), a number of conductive paths 103, 104 penetrating an insulating resin sheet 100 in the thickness direction of the sheet have been formed, wherein the conductive paths are insulated from each other.
The conductive paths include a through-hole filled with a good conductor metal, a through-hole having a metal conductive wire penetrating the insulating resin sheet, as shown in FIG. 6(c), and conductive magnetic particles locally collected together to form a conductive path as shown in FIG. 6(d).
Conventionally, the following production method is used for producing the anisotropic conductive sheet shown in FIG. 6(d).
First, as shown in FIG. 7(a), a flowable insulating resin 101a is placed in a sheet-like molding die (sheet-like cavity space defined by partition plates 201, 202 in the Figure). The insulating resin 101a contains conductive magnetic particles 102 in a uniformly dispersed state.
Next, magnets M1 and M2 are placed in positions facing each other relative to the insulating resin in the die, where the conductive path is to be formed, and a local magnetic field is allowed to act on the sheet in the thickness direction (magnetic field lines are shown with a broken arrow in the Figure). As shown in FIG. 7(a), uniformly dispersed conductive magnetic particles 102 are attracted toward the local magnetic field and, as shown in FIG. 7(b), are collected together to form a conductive path.
Finally, the insulating resin is cured (set) to give an anisotropic conductive sheet.
However, the present inventors considered the above-mentioned conventional production methods in more detail and found the following problems.
As shown in FIG. 7(a), the conductive magnetic particles 102 can be attracted toward the central direction when a local magnetic field is applied to the insulating resin in the die, since the magnetic field between the opposing magnets is extending in the initial stage.
As the conductive magnetic particles 102a are collected together as a conductive path, as shown in FIG. 7(b), the conductive path becomes a magnetic path and, since the magnetic field is confined in the conductive path, the magnetic field can hardly act on the surrounding area. As a result, a lot of conductive magnetic particles 102b are left dispersed in the insulating resin, as shown in FIG. 7(b). The conductive magnetic particles 102b left behind are not attracted to the conductive path, and merely disperse in the film without contributing to the conductive path.
Therefore, the problem that the present inventors found in the conventional production methods is that a portion of the conductive magnetic particles is left dispersed in the insulating resin instead of being incorporated into the conductive paths.
Since conductive magnetic particles are left in the insulating resin, problems occur in the anisotropic conductive sheets produced by conventional methods. More specifically, (a) a preferable conductive path having a sufficient sectional area cannot be obtained and (b) reliability in the insulation between conductive paths is impaired due to the remaining conductive magnetic particles.
Even if a greater amount of conductive particles is dispersed in the insulating resin to solve the above-mentioned problem (a), the number of the remaining conductive magnetic particles increases and the problem of reliability relating to the above-mentioned (b) becomes greater.
It is therefore an object of the present invention to solve the above-mentioned problems and reduce the number of conductive magnetic particles left in the insulating resin in the production of an anisotropic conductive sheet comprising conductive magnetic particles locally collected together.